For unfinished buildings, look for a ladder or piece of scaffolding that is on an unreachable place (for example, on top of other object, be it a house, a berry bush, a container…). Make a ladder so that hearthlings can reach it and remove it.

For all that scaffolding… if you are tired of selecting individual ladders and destroying them, go to a previous savefile if you have it.

If you have one of the building not being built (before trying to remove it), you can upload it and we can find where they are stuck.